In principle, the "defect" referred to herein means every kind of defect, such as adhering foreign material deteriorating the quality of the inspected part and defective pattern. Where "foreign material" and a "defect" are used in the same sentence or paragraph, the former "foreign material" means a defect caused by adhesion of foreign material, while the latter "defect" means a defect due to other than adhesion of foreign material. The "defective pattern" referred to herein means a defect in a pattern formed on a part under inspection and does not embrace any defect such as adhesion of foreign material.
A part-inspecting system (technique J01) for performing detailed inspection according to information obtained by a preliminary inspection as illustrated in FIGS. 72-76 is known as a part-inspecting system of this kind. This system performs a preliminary inspection, using a commercially available optical part-inspecting apparatus. Information about the results of the preliminary inspection, such as the positions of defects on the inspected part and their size, is stored in memory. Then, the system performs a review inspection, or detailed inspection, with a review apparatus, by reference to the information obtained by the preliminary inspection. A review SEM (scanning electron microscope), an optical review apparatus using an optical microscope, or the like is used as the aforementioned review apparatus.
It is to be noted that "review" referred to herein is to perform detailed inspection of an object under inspection, for knowing the exact position of any defect or foreign material, its shape, or distribution. The "review SEM" using an electron microscope comprises a review SEM body and an EWS (engineering workstation) interfaced to the body by a communications cable.
The controller of the review SEM and the EWS are connected by a communications cable and housed in a common housing. The EWS connected with the SEM controller might be referred to as the SEM EWS. A display device DE for the SEM EWS and a display device D for the SEM controller are both held to the housing described above.
FIG. 72 schematically illustrates a conventional part-inspecting inspecting system of the above-described kind. An optical foreign material-inspecting apparatus 01, an optical defect-inspecting apparatus 02, a defect image filing system (DIFS) server 03 for storing information, an EWS (engineering workstation), etc. are connected by a network N such as Ethernet.
The optical foreign material-inspecting apparatus 01 and the optical defect-inspecting apparatus 02 detect foreign materials and defects on an inspected part and produce data regarding the positions and sizes of the detected foreign materials and defects. The outputs from these two kinds of apparatuses are similar in format. Accordingly, the optical foreign material-inspecting apparatus 01 and the optical defect-inspecting apparatus 02 are collectively referred to as the preliminary inspecting equipment (01, 02).
When a part to be inspected (i.e., a bare wafer on which no pattern is formed) is placed in position for inspection, the optical foreign material-inspecting apparatus 01 automatically detects the position and the size of adhering foreign material. Surfscan 6600 and Surfscan 7700 manufactured by TENCOR Corporation are available as such foreign material-inspecting apparatus. As a result of the inspection performed by the inspecting apparatus 01, a foreign material information file holding the position and the size of any foreign material and other data is delivered.
When a part to be inspected (i.e., a wafer on which a pattern is formed) is placed in position for inspection, the optical defect-inspecting apparatus 02 automatically detects the size and position of any defect (such as adhering foreign material, defective pattern, flaw, or the like). Apparatus 21XX manufactured by KLA Corporation is commercially available as this defect-inspecting apparatus 02.
The preliminary inspecting equipment (01, 02) consisting of the optical foreign material-inspecting apparatus 01 and the defect-inspecting apparatus 02 delivers foreign material information files and defect information files, which might be collectively referred to as preliminary inspection information files.
The preliminary inspection information files created by the optical foreign material-inspecting apparatus 01 and the defect-inspecting apparatus 02 are stored in computers ancillary to the optical foreign material-inspecting apparatus 01 and the defect-inspecting apparatus 02, respectively, or in the DIFS server 03.
Product serial numbers, lot numbers, wafer identification numbers or codes, data about process steps, data about the fabrication equipment, and date are stored in the preliminary inspection information files. Besides, the number of foreign materials, the number of defects, their positions on the wafer, their sizes, and other data are stored. The preliminary inspection information stored in the preliminary inspection information files can be displayed as shown in FIGS. 73A and 73B, for example.
FIGS. 73A and 73B show an example of preliminary inspection information displayed. FIG. 73A shows the contour of a wafer under inspection, as well as the positions of foreign materials or defects on the inspected part. FIG. 73B is a list of numbers given to the foreign materials or defects, their positions, their sizes, and other kinds of information. The preliminary inspection information permits one to grasp the situation and tendency of occurrence of defects produced during fabrication of inspected parts. Therefore, a preliminary inspection information file such as a foreign material information file or defect information file is indispensable for a yield management system.
More specifically, a semiconductor fabrication sequence fabricates 200 to 300 semiconductor chips on a bare wafer. If the result of a preliminary inspection needs an accurate inspection, each inspected part (such as a bare wafer or wafer on which semiconductor chips are being produced) is inspected accurately. If foreign materials or defects discovered by the accurate inspection are considered to degrade the quality of the inspected part below the acceptable level, then it is necessary to find and remove the cause of the defects.
If the operator can infer from the information about the preliminary inspection that the process sequence is at fault, the review SEM is used to know, or review, the shapes of foreign materials or defects or the circumstance. The review SEM can perform a quick review by using the information contained in the preliminary inspection information, i.e., the positions and sizes of the foreign materials or defects. A review SEM using a scanning electron microscope or an optical review SEM can be employed as the above-described review SEM.
When an inspected part should be reviewed by the use of the review SEM, the sample stage (not shown) of the review SEM is set on the part. Then, the initial magnification of the review SEM is set to 3000.times., for example. Information (hereinafter referred to as the "optical inspection information") about the positions and sizes of foreign materials or defects previously obtained by the optical foreign material-inspecting apparatus 01 or the defect-inspecting apparatus 02 is read into the SEM EWS. At this time, images shown in FIGS. 73A and 73B are displayed on the display device DE connected with the SEM EWS.
The operator watches the images of FIGS. 73A and 73B, lists the numbers given to foreign materials or defects that might adversely affect the quality of the inspected part, and manually specifies the numbers given to the foreign material or defect that he or she wants to review.
The sample stage of the review SEM is moved according to the information about the position of the foreign material or defect bearing the specific number. The inspected part is moved so that an SEM image of the specified foreign material or defect is displayed in the middle of the display device D.
Then, an electron microscope image of the foreign material or defect specified by the review SEM is displayed on the display device D. At this time, if the position of the foreign material or defect found by the preliminary inspection using an optical microscope agrees with the coordinates of the foreign material or defect on the inspected part set on the sample stage of the review SEM, then it follows that the specified foreign material or defect is displayed in the center of the display device D. If they do not agree, it is necessary to correct the X- and Y-coordinates of the sample on the sample stage of the review SEM.
Generally, the position of the foreign material or defect found by the preliminary inspection using an optical microscope deviates from the coordinates of the foreign material or defect on the inspected part set on the sample stage of the review SEM. Therefore, the specified foreign material or defect is not first displayed in the center of the display device D. Normally, as shown in FIG. 74A, the foreign material or defect is displayed off the center of the viewing screen of the display device D. For example, the magnification of the foreign material or defect shown in FIG. 74A is 3000.times.. Where the image is magnified at such a magnification (say, 1000.times. or 3000.times.) while the foreign material or defect is off the center of the viewing screen, the magnified foreign material or defect is outside the viewing screen and hence impossible to observe. Therefore, where the foreign material or defect is magnified at a high magnification for observation, it is necessary to bring the displayed foreign material or defect in the center of the viewing screen of the display device D.
FIGS. 74A-74C illustrate SEM images of a foreign material or defect specified by the operator and displayed on the display device D. FIG. 74A shows a condition obtained immediately after the operator makes a designation. FIG. 74B shows a condition in which the foreign material or defect is brought into the center of the viewing screen of the display device D. FIG. 74C shows the image of FIG. 74B on a magnified scale.
When an image shown in FIG. 74A is displayed, the operator moves the center of a crisscross cursor Dk into the center of the foreign material or defect displayed together with the cursor, using a mouse. Then, he cricks on the left button. The sample stage (not shown) moves a distance corresponding to the amount of movement of the cursor Dk. As a result, the foreign material or defect moves into the center of the viewing screen of the display device D, as shown in FIG. 74B.
Under the condition of FIG. 74B, the image of the foreign material or defect is magnified, thus obtaining an image as shown in FIG. 74C. Then, the operator observes the displayed foreign material or defect. Thus, he can find the cause or kind of the foreign material or defect in the inspected part, identify the cause of the defect, and make a decision as to whether the defect is fatal.
The operator observes the inspected part in a two-dimensional plane in the sequence illustrated in FIGS. 74A-74C. If it is impossible to precisely judge the foreign material or defect on the part under inspection, the part may be rotated about a vertical line or tilted about a horizontal axis during observation.
Observation of Rotated Image
FIGS. 75A-75E illustrate the case where an observation is made while the foreign material or defect shown in FIG. 74C is tilted at an angle of 60.degree. about a vertical axis parallel to the electron beam of the review SEM. FIG. 75A shows a state in which the angular position .theta.=0.degree. (i.e., the state of FIG. 74C). FIG. 75B shows a state obtained by rotating the state of FIG. 75A through 10.degree. (i.e., .theta.=10.degree.). FIG. 75C shows a state obtained by translating an inspected part-holding member in the state of FIG. 75A so that the foreign material or defect is brought into the center of the viewing screen of the display device D. FIG. 75D shows a state obtained by rotating the state of FIG. 75C through 10.degree. (i.e., .theta.=20.degree.). FIG. 75E shows a state obtained by translating the inspected part-holding member in the state of FIG. 75C so that the foreign material or defect is brought into the center of the viewing screen of the display device D.
Where the operator wants to observe the foreign material or defect shown in FIG. 75A after rotating it through 60.degree., if the foreign material or defect is placed in the center of rotation, then the foreign material or defect will not move out of the field of view even if the foreign material or defect in the state of FIG. 75A is directly rotated through 60.degree.. However, when the foreign material or defect is off the center of rotation, if the foreign material or defect is rotated through 60.degree., it will move out of the field of view, making the observation impossible.
Accordingly, the inspected part-holding member is first rotated through only 10.degree.. The foreign material or defect shown in FIG. 75A is moved off the center of the field of view and takes the state of FIG. 75B. Under this condition, the holding member is translated to bring the foreign material or defect into the center of the field of view as shown in FIG. 75C.
Then, the inspected part-holding member is further rotated through 100 until .theta.=20.degree.. The foreign material or defect shown in FIG. 75C moves off the center of the field of view and assumes the state of FIG. 75D. The holding member is translated to move the foreign material or defect into the middle of the field of view as shown in FIG. 75E.
The operator repeats these operations until the angular position reaches 60.degree.. Under this condition, he can observe the foreign material or defect to review the cause of the generation of the foreign material or defect or its kind.
Observation of Tilted Image
FIGS. 76A-76E illustrate the case where an observation is made after tilting the image of the foreign material or defect shown in FIG. 74C by 45.degree. around a horizontal axis vertical to the electron beam of the review SEM. FIG. 76A shows a state in which the tilt angle .phi.=0.degree. (i.e., the state of FIGS. 74C and 75A). FIG. 76B shows a state obtained by tilting the state of FIG. 76A by 5.degree. (i.e., .phi.=5.degree.) FIG. 76C shows a state obtained by translating the inspected part-holding member in the state of FIG. 76B so that the foreign material or defect is brought into the center of the viewing screen of the display device D. FIG. 76D shows a state obtained by tilting the state of FIG. 76C by 5.degree. (i.e., .phi.=10.degree.). FIG. 76E shows a state obtained by translating the inspected part-holding member in the state of FIG. 76D so that the foreign material or defect is moved into the center of the viewing screen of the display device D.
When the operator wants to make an observation after tilting the image of the foreign material or defect shown in FIG. 76A at an angle of 45.degree. (.phi.=45.degree.), the image is not tilted at 45.degree. at once, for the same reason as described in connection with FIGS. 75A-75E. Rather, the tilt angle is increased in increments of 5.degree., for example. At each tilt angle, the foreign material or defect is moved into the center of the field of view. These operations are repeated. In this way, the operator can observe the foreign material or defect tilted at an angle of 45.degree. (i.e., .phi.=45.degree.).
Where the inspected part is tilted as shown in FIGS. 76A-76E, as the distance between the foreign material or defect on the inspected part held on the holding member and the tilting axis of the holding member increases, the amount of deviation of the position of the foreign material or defect produced per unit tilt angle increases. Therefore, it is necessary to reduce the increment .phi. of the tilt angle according to the distance.
The operator classifies the foreign materials or defects from various points of view (e.g., causes of foreign materials or defects, shapes, and states), using the review SEM, according to the results of the review obtained by the method illustrated in connection with FIGS. 73A-76E. The results of the sorting are used as important information in a yield-managing system.
The prior art part-inspecting system makes use of the following known technique (J02) for measuring the dimensions of patterns on inspected parts by a length-measuring SEM. In order to monitor the processed part under inspection during a lithography or etching step of a semiconductor wafer fabrication sequence, this known technique uses the length-measuring SEM. Linewidths at certain measuring points on the wafer and hole diameters are monitored, using the length-measuring SEM. Hence, any process abnormality can be detected. As a result of the measurement, data about the measured lengths are produced. This data contains product serial numbers, lot numbers, wafer identification numbers, data about process steps, data about the fabrication equipment, and date. In addition, the number of measured points, the positions of the measured points, and linewidths are contained. The data about the measured lengths permits the operator to grasp the state and tendency of the fabrication process. Consequently, data about the measured lengths are indispensable for the production management system.
Problems with Technique (J01)
With the technique (J01) described above, the following problems take place:
(a) Since a review is made with a review SEM, it is necessary for the operator to specify individual defect locations, to enter them in a list, and to specify portions to be reviewed. Hence, the operator must always participate in these operations.
(b) Where foreign materials and defects are identified and classified according to the results of observations, the operator must participate in these operations. Since differences are produced among individual human operators, the result of sorting will differ among the individual operators.
(c) In performing observations, it is necessary to rotate and tilt the sample stage. If these operations are carried out, the field of view escapes. In consequence, the operator is required to perform a centering operation.
(d) In performing observations, the magnification needs to be modified to an appropriate value. Again, an operator's operation is necessary.
(e) Where the results of a review, or detailed inspection, are classified and stored in the DIFS server, the result of sorting differs among individual operators, because they differ in amount of knowledge and capability, and because intuitive judgments are often made. As a result, the reliability of the sorting tends to be impaired.
Problems with the Technique (J02)
(f) Since the length-measuring SEM is unable to tilt inspected parts, the instrument is limited to two-dimensional measurements, such as of linewidths as viewed from just above and of hole diameters. Step heights, heights, depths, film thicknesses, or the like cannot be measured during processing steps.